The loop gain in an oscillator is determined by all of its elements. If a differential amplifier of the limiter of an oscillator has a reasonably well defined gain which it does if it is an operational amplifier with a large amount of feedback, and if you define that gain to be, for example, 2, and if the feedback network has a gain of slightly more than 1/2 in a loop gain of 1.01, and if the loop gain is positive, oscillation will begin. For oscillation to begin a gain of at least 1 is required. However, many elements can vary. To compensate for various manufacturing tolerances extra gain is given, for example, 1.5; the oscillation will then start and it will start to grow because it will stay stable at a specific level only if the gain is exactly one. The oscillation will start to grow and something must happen to reduce that gain down from 1.5 to 1.0. After the gain is reduced to one, the oscillator will then maintain the level at 1. Usually the level of oscillation is maintained at 1 because the signal runs into the power supply of the oscillator. Therefore the power supply must be well defined and the clipping point of the amplitude of the signal must be symmetrical on both of the positive and negative sides. All oscillators start with a little extra gain. Preferably, an oscillator will start off with as little extra gain as possible. Therefore, it is necessary that the components of the oscillation circuit be controlled very well.
Another manner in which one can control the level of the signal is by means of a feedback circuit. A feedback circuit senses the amplitude of the signal of an oscillator with a precision rectifier and when the desired level of the signal is obtained the signal is sent back to a non-linear element which controls the gain of the amplifier. However, this feedback technique is slow because the non-linear element must not be non-linear to the oscillating frequency, it must only be non-linear in a dc sense which results in the feedback being slow, otherwise distortion would result.
Limiter circuits which limit the amplitude of alternating current in oscillators are well known. An example of a limiter circuit is shown in FIG. 1. In the limiter circuit shown in FIG. 1 a Zener diode is surrounded by a bridge rectifier. The circuit clips both sides of the signal reasonably well but it is dependent on the diode voltages. If the diodes were ideal, namely at a voltage of +0.0001 V they are on and at a voltage of -0.0001 V they are off and if the Zener diode had a tolerance of 0.0001% then the circuit would be fine, it would work great. However, in practice, Zener diodes have a tolerance of 1-5% at best and the temperature coefficient is undesirable. Resistance of resistor R in the circuit of FIG. 1 is much lower than the OFF resistance of diodes D1 through D4, so the signal is passed undistorted to the output when the diodes are OFF. Resistance of resistor R is much larger than the ON resistance of diodes D1 through D4 and of reference diode RD, so Voltage Vout=Vrd+Vd1+Vd 3 for a positive signal and Vout=-Vrd-Vd2-Vd4 for a negative signal when the diodes are ON. The diode voltages Vrd+Vd1+Vd3, however, cannot be made precisely equal to Vrd+Vd2+Vd4 because no two components have identical characteristics. D1 through D4 could be matched discretely or they could be manufactured on the same silicon die to reduce variations in characteristics, but neither of these methods completely solve the problem.
A diode is by itself a rectifier but it is not ideal so as you go above zero it starts to turn on, but very slowly. To remedy this a diode is place in the feedback loop of an amplifier. The gain of the amplifier pushes the signal closer to zero thus making the circuit more ideal. That is the basis behind precision rectifiers.
A clamp is also a diode type circuit but there is a resister and a diode and the diode simply limits, because the diode is biased at a voltage other than zero volts. A classic method to reduce dependence on component variations is to put the components in the feedback loop of an operational amplifier. If this is done, the effect of the component variations is reduced in magnitude by the gain of the amplifier. Amplifier gains can be 100,000 thereby reducing the error caused by component variations to 1/100,000 namely to 0.001%. An example of an active clamp circuit of the prior art using the above-described technique is shown in FIG. 2. Such a circuit will pass the signal to the output undistorted whenever the signal voltage is below +Vref and will output a fixed voltage of +Vref whenever the signal voltage is greater than or equal to the clamp amplitude (+Vref). Clamping occurs precisely at +Vref regardless of variations in characteristics of diode D. However, clamping is one sided. Negative signal voltages are not clamped, rather, they pass undistorted for all amplitudes as is shown in the graph of FIG. 2b. Also the clamp has a problem in that the amplifier saturates when the diode system is off. Saturation of a differential amplifier occurs when the voltage of one input departs appreciably from the voltage of the other input resulting in the output voltage approaching the supply voltage. Saturation is detrimental to a system because when the input voltage departs appreciably from the voltage of the other input there is a large accumulation of charge inside the differential amplifier. Before the clamp can be turned back on the accumulated charge inside the differential amplifier must be discharged. In other words, a long recovery period to unsaturate the amplifier is required when the system is turned on again. This can be a problem in those circumstances which require immediate response of the system when the system is turned on.